The present invention relates to a method of manufacturing an electrophotographic photoreceptor, more particularly to a method of manufacturing a photoreceptor of amorphous silicon having low residual voltage or potential and high charging characteristic or capability.
The electrophotographic photoreceptor for use in a copying apparatus or a printer untilizes Se-Te, Se.sub.2 As.sub.3, ZnO or other inorganic materials. These materials, particularly Se system are suitable only at wavelengths in the range of below 650-700 nm and are deteriorated at 50.degree. C.
In order to improve these defects the photoreceptor of amorphous silicon (hereinafter referred to as a-Si) has been developed. Such an a-Si has high sensitivity at longer wavelength and is hard, stable at a temperature of about 200.degree. C. and not toxic, so a-Si is suitable for use in the photoreceptor. This photoreceptor consists of only one a-Si layer provided on a base layer. The a-Si layer is generally formed by glow discharge decomposition of monosilane (SiH.sub.4) and has low resistivity such as 10.sup.10 .OMEGA.cm or less so that charges cannot be held at the surface of the photoreceptor.
In order to obtain necessary charging characteristic or capability of the photoreceptor there is utilized a method of making the a-Si layer high resistivity by doping of oxygen or nitrogen. However, in this method, it is difficult to control the doping concentration of oxygen or nitrogen for obtaining an optimum resistivity.
Accordingly, there is utilized another method of preventing injection of charges into the a-Si layer as the sensitized layer from the surface thereof and the substrate by providing at the interface with the substrate or the surface of the a-Si layer a layer of high resistive material such as a-SiC, a-SiN.sub.x, a-SiO.sub.x or a a-Si layer of n or p conductivity type according to negative or positive surface charges. According to the latter method, the photoreceptor consists of multilayers on the base layer and the charge preventing layer has a thickness of few hundreds .ANG. to few .mu.m.
In the above latter method two conditions are required to the intermediate a-Si layer serving as the sensitized layer; firstly when no light is incident on the photoreceptor, slight thermal carriers are generated and dark decay time is long. Secondly when light is incident on the photoreceptor, it is necessary to transport the generated photo carriers to the interface as soon as possible without capturing them on the way. When the a-Si layer or film is formed by plasma CVD (glow discharge process), however, the first and second conditions are incompatible with each other, since in case of utilizing the a-Si layer as an electrophotographic photoreceptor, its thickness is made 20 .mu.m or more in order to obtain a sufficient charging characteristic resulting in a necessity of growing the a-Si layer at a high rate (.about.10 .mu.m/h or more) so that a large number of defects such as trapping level, dangling bond, microvoids or the like due to SiH.sub.n (n=2, 3) are caused as compared with the a-Si layer formed with low rate which is utilized in a solar cell or photo sensor. As a result, the first condition is satisfied but the second condition does not satisfy. In the case of a positive charging system, particularly, if a deep trapping level, for holes is present in the a-Si layer holes trapped at the deep level find difficulty in reexciting due to heat and this becomes a cause of high residual potential.